Method for identifying and correcting error in a central processing unit

ABSTRACT

A central processing unit (CPU) repeatedly interrupts execution of software to save the CPU state, i.e. contents of various storage elements internal to the CPU, until an error occurs during the execution. On occurrence of the error, the CPU once again saves state and only then passes control to a handler in the software for handling the error. The state saving steps can be implemented in a computer process by use of a timer interrupt or by use of system management, or ICE breakpoint instructions that are included in the x86 instruction set. Errors can be debugged off-line in a development system, for example, by use of an in-circuit emulator to load the saved CPU states sequentially into the development system, thereby to recreate the error condition. Errors can also be debugged proactively, even before the error occurs, by use of a number of known-to-be-erroneous instructions and corresponding fix instructions. For proactive debugging, the CPU compares instructions to be executed with each of the known-to-be-erroneous instructions, and on finding a match, injects the corresponding fix instructions into the to-be-executed instructions. Therefore, known errors e.g. the PENTIUM arithmetic bug are avoided, e.g. by replacing one arithmetic instruction with another arithmetic instruction. Moreover, if an error has not yet been debugged, a temporary fix instruction can be used to gracefully terminate an application.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to and incorporates by reference herein in their entirety the following commonly owned, concurrently filed, copending U.S. Patent Applications: (1) “A Port For Fine Tuning A Central Processing Unit” by Sherman Lee and David G. Kyle, application Ser. No. 08/710,337, now issued as U.S. Pat. No. 5,937,203; and (2) “A Method For Fine Tuning Operation of Circuitry In a Central Processing Unit” by Sherman Lee and David G. Kyle, application Ser. No. 08/710,294, now issued as U.S. Pat. No. 5,812.425. This application is a continuation of application Ser. No. 08/710,336 entitled “A Method for Identifying and Correcting Errors in a Central Processing Unit” by Sherman Lee and David G. Kyle filed Sept. 16, 1996, now issues as U.S. Pat. No. 6,202,174 B1 which is incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to a method for identifying errors in a programmed digital computer and for correcting the identified errors. In particular, this invention relates to a method for monitoring instructions and data that cause errors, analyzing the monitored instructions and data to predict errors and for preventing future errors from occurring, for example by inserting corrective software.

BACKGROUND OF THE INVENTION

MICROSOFT Corporation's Dr. Watson is a debugging tool that logs information regarding internal operations of the operating system “WINDOWS” into a failure report. Dr. Watson logs the information after any application software (typically called just “application”) encounters an error, that MICROSOFT calls “unrecoverable application error (UAE).” See, for example,. “An Annotated Dr. Watson Log File,” KB:Windows SDK KBase, Microsoft Development Library, MICROSOFT Corporation, One Microsoft Way, Redmond, Wash.; “Postmortem Debugging,” Matt Pietrek, Dr. Dobb's Journal, September 1992; and “Exception Handlers and Windows Applications,” Joseph Hlavaty, Dr. Dobbs Journal, September 1994; all of which are incorporated by reference herein in their entirety.

Briefly, a Dr. Watson failure report contains information on (1) the name of an application that failed, (2) the error encountered, such as “Exceed Segment Bounds (Read),” (3) the instruction's address at which the failure occurred, (4) the instruction that caused the failure, (5) the contents in various registers, such as CPU registers, instruction pointer (also called “program counter”), stack pointer, base pointer, code segment selector, stack segment selector, data segment selector, extra segment selector, 32-bit registers and flag bits (e.g. Overflow bit, Direction bit, Sign bit, Zero bit, Carry bit, Interrupt bit, Auxcarry bit and Parity bit), (6) WINDOWS installation and environment information, (7) stack frame information such as disassembled instructions surrounding the failed instruction, and several levels of nested function calls leading to the failed instruction, (8) names of all tasks when the failure occurred and (9) user response typed into a “Dr. Watson's Clues” dialog box.

MICROSOFT Corporation recommends that a user exit WINDOWS after a UAE occurs, and if exiting is not possible, to restart the personal computer. See “The DrWatson and MSD Diagnostics,” KB:Windows 3.x KBase, Microsoft Development Library, MICROSOFT Corporation, One Microsoft Way, Redmond, Wash., also incorporated by reference herein in its entirety. MICROSOFT Corporation further recommends that after a UAE occurs, the user should run MICROSOFT DIAGNOSTICS (MSD) that identifies system configuration information, such as the BIOS, video card type, manufacturer, installed processor(s), I/O port status, operating system version, environment settings, hardware devices attached, and additional software running concurrently with MSD. Id. All of these actions can result in loss of valuable data, as well as valuable time before a user can continue using the application.

MICROSOFT Corporation also recommends that after logging several UAES, the user should send the log to MICROSOFT Corporation, although MICROSOFT Corporation cannot respond to log contributors. Id. Therefore, the user receives no assistance in identifying the problem that caused the UAE and in fixing the application to avoid that particular UAE in future. Moreover, Dr. Watson appears to log only an application's UAEs failures, and cannot be used for debugging other errors, such as errors in the operating system or errors in hardware.

Errors in hardware can be debugged using a built-in “debug” part of the type present in INTEL's P6 (also called “Pentium Pro”) microprocessor. INTEL recommends the P6's debug port as an aid for designing a system board on which the CPU is mounted. See, for example, “Intel equips its P6 with test and debug features,” Electronic Engineering Times, Oct. 16, 1995, n870, pages 1-2, that is incorporated by reference herein in its entirety.

Briefly, the P6 debug port is typically connected to an “in-target probe” (ITP) via a 30-pin connector, and allows access to boundary-scan (JTAG) and built-in-self-test (BIST) structures on the P6 microprocessor. Through an ITP such as ICE-16 available from, for example, American Arium, Tustin, Calif., board designers can control program execution, set break points, monitor the P6's access of registers, memory and input-output devices.

However, a typical user neither has access to an ITP nor the expertise needed to use the ITP. Therefore, the user is still unable to identify the problem that causes a UAE and unable to fix the application to avoid known UAEs in future.

SUMMARY

In accordance with the invention, a central processing unit (CPU) repeatedly interrupts execution of software to save the CPU state, i.e. contents of various storage elements internal to the CPU, until an error occurs during the execution. On occurrence of the error, the CPU once again saves state and only then passes control to a handler in the software for handling the error. Each time the CPU state is saved at locations in memory different from the previous time so that a sequence of CPU states is saved when control passes to the handler. The storage elements whose contents are saved can be of two types: (1) accessible, and (2) inaccessible to the executing software, such as an operating system or an application. Moreover, the above-described state saving steps can be implemented, in different embodiments of the invention, in hardware (e.g. as a state machine) or in software (e.g. in basic-input-output-system (BIOS), in an operating system, as a device driver, or as a utility). In one specific embodiment, the state saving steps are implemented in a computer process by use of x86 instructions.

¹ The x86 instruction are instructions executable by microprocessors compatible with microprocessors in the 8086, 80286, 80386, 80486, Pentium and Pentium Pro (P6) families of microprocessors available from Intel Corporation, Santa Clara, Calif.

In one embodiment, errors are debugged off-line in a development system, for example, by use of an in-circuit emulator to load the saved CPU states sequentially into the development system, thereby to recreate the error condition. If the frequency of the saved CPU states is too coarse to find the source of the error, the CPU states can be saved more frequently, e.g. after shorter time periods, on every jump instruction, on every input-output instruction, on every function-call instruction, or on some combination these events, depending on one or more flags. The flags can be set, for example, in a configuration file that is checked at the startup of the computer process. The sequence of saved CPU states allows recreation of error conditions otherwise not possible in the prior art. Moreover, the CPU states are saved transparent to the software, thereby allowing recreation of errors in an operating system as well as errors from interaction between the operating system and an application, both of which were not possible in the prior art.

In accordance with the invention, an error can also be debugged proactively by a computer process, even before the error occurs, by use of a number of known-to-be-erroneous instructions and fix instructions corresponding to the known-to-be-erroneous instructions. In one embodiment, the CPU compares instructions to be executed with each of the known-to-be-erroneous instructions, and on finding a match, injects the corresponding fix instructions into the to-be-executed instructions. In this embodiment, these proactive error debugging steps are executed by the state saving process optionally depending on a flag that is set or cleared, for example, in a configuration file. In another embodiment, the proactive error debugging steps are implemented in a different process that executes independent of the state saving process, i.e. does not save CPU states.

Therefore, well known errors e.g. the 80286 jump bug or the PENTIUM arithmetic bug are easily avoided, e.g. by inserting a no-op instruction before a jump instruction or by replacing one arithmetic instruction with another arithmetic instruction. Such proactive debugging allows a user to continue to use, for example, a defective PENTIUM or defective software and not have any known errors. Moreover, if an error has not yet been debugged, the handler can add an erroneous instruction to the known-to-be-erroneous instructions with a corresponding temporary-fix instruction to gracefully terminate the application, e.g. if the erroneous instruction is known to crash (e.g. “freeze”) the CPU. Such graceful termination of the application allows the CPU to continue execution of other software that may be of value to a user, e.g. to eliminate the need to reboot the operating system otherwise required in the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C each illustrate a central processing unit (CPU) circuitry tuner in three different embodiments of a computer system.

FIG. 2 illustrates, in a high level flowcharts and block diagrams, various steps performed by the CPU-circuitry tuner of FIGS. 1A-1C.

FIG. 3 illustrates addresses and contents of main memory 120 and system management memory 130 in one embodiment of the invention.

DETAILED DESCRIPTION

In accordance with the invention, a central processing unit (hereinafter “CPU”) repeatedly interrupts execution of software to save the CPU state, i.e. contents of various storage elements internal to the CPU, until an error occurs during the execution. On occurrence of the error, the CPU once again saves state and only then passes control to a handler in the software for handling the error. The storage elements whose contents are saved can be of two types: (1) accessible, and (2) inaccessible to the executing software, such as an operating system or an application. The state saving steps can be implemented in a computer process by use of system management instructions that are included in the x86 instruction set. Errors can be debugged off-line in a development system, for example, by use of an in-circuit emulator to load the saved CPU states sequentially into the development system, thereby to recreate the error condition. Errors can also be debugged proactively, even before the error occurs, by use of a number of known-to-be-erroneous instructions and fix instructions corresponding to the known-to-be-erroneous instructions. Specifically, the CPU compares instructions to be executed with each of the known-to-be-erroneous instructions, and on finding a match, injects the corresponding fix instructions into the to-be-executed instructions. Therefore, known errors e.g. the PENTIUM arithmetic bug are avoided, e.g. by replacing one arithmetic instruction with another arithmetic instruction. Moreover, if an error has not yet been debugged, a temporary fix instruction can be used to gracefully terminate an application that would otherwise “freeze” the CPU.

In one embodiment, the above-described CPU is a “tunable CPU” having a number of tunable units, with each tunable unit having one or more parameters that can be changed, to thereby change circuitry in the CPU. Examples of circuitry in a tunable unit and a device (called “fine tuning port”) for changing circuitry in the CPU are described in the above-incorporated application Ser. No. 908/710,337.

In this embodiment, the above-described state saving steps are implemented in a computer process that fine tunes the tunable CPU. The computer process waits for a triggering event indicating the need for fine tuning of the tunable CPU, reads one or more statistics on performance of the tunable CPU, compares the read statistics with predetermined statistics patterns to determine one or more parameter signals, and drives the parameter signals to one or more tunable units, thereby to change the circuitry in the tunable CPU. Examples of steps for fine tuning the tunable CPU are described in the above-incorporated application Ser. No. 08/710,294.

Although specific circuitry is described herein for a particular embodiment of the invention, other such embodiments will be obvious to a person skilled in the art of designing debugging systems for microprocessors in view of the disclosure. Accordingly, the embodiments described herein are merely illustrative and not limiting.

In one embodiment, a computer system 100A includes a CPU 110A having a built-in CPU state saver 110S. CPU state saver 110S repeatedly saves the state of CPU 110A, such as contents of storage elements 110D-110N as described more completely below. In addition to CPU 110A, computer system 100A includes a read only memory 111 that is encoded with basic input output system instructions for CPU 110A. Read only memory 111 and CPU 110A are coupled to each other by a CPU bus 112 that is also coupled via a system bridge (not labelled) to a system bus 113. System bus 113 in turn is coupled to a floppy drive 114, a hard disk 115, a monitor 116, a keyboard 117 and a mouse 118. CPU bus 112 is also coupled to a main memory 120 that is encoded with a number of applications, e.g. application 120A . . . 120I . . . 120M, where M is the number of applications.

In addition to the above-described parts, computer system 100A also includes a system management memory 130 that is coupled via CPU bus 112 to CPU 110A, and that is accessible to CPU state saver 110S. System management memory 130 is typically “hidden” from i.e. made inaccessible from applications 120A-120M in the manner described in, for example, “The CPU and Undocumented Instructions,” Chapter 3 of the book “The Undocumented PC” by Frank van Gilluwe, Addison-Wesley Publishing Company, Reading, Mass.; this book is incorporated by reference herein in its entirety. In this embodiment, system management memory 130 holds a number of CPU states e.g. CPU state 130A, . . . 130I, . . . 130N, where N is the number of CPU states. CPU states for one embodiment are described below in reference to FIG. 3.

In this embodiment, CPU state saver 110S in CPU 110A is implemented in hardware as circuitry that performs a number of steps illustrated in FIG. 2 (described below). In an alternative embodiment, CPU state saver 110S is encoded as microcode resident in storage elements (not shown) in CPU 110A that also performs the steps of FIG. 2. In another embodiment, a CPU 110B (FIG. 1B) does not have the above-described CPU state saver 110S, but rather has a software version, CPU state saver 1115 that is encoded in read only memory 111 in addition to the Basic Input Output System (BIOS) instructions. CPU state saver 111T also performs the steps described below in reference to FIG. 2. Note that the same reference numerals are used for various parts in FIGS. 1B and 1C that are similar or identical to the corresponding parts in FIG. 1A.

In yet another embodiment of the invention, both CPU 110C (FIG. 1C) and read only memory 111C do not have a CPU state saver 110S or 111s. Instead, a software version, CPU state saver 120S is encoded in main memory 120C. In addition, computer system 100C does not have system management memory 130, and rather CPU states 130A-130N are saved in main memory 120C. In this embodiment as well, CPU state saver 120T performs the steps described below in reference to FIG. 2.

A CPU state saver 210 in one embodiment of the invention initializes variables on start up in a step 211 and thereafter checks in step 212 whether the state of CPU 110A (FIG. 1A) can be saved, and if not stops in step 213. Step 212 can be implemented, for example, by determining the identity of CPU 110A and jumping to instructions specific to the determined identity for the following steps. The identity of CPU 110A can be determined, for example, as described in “System Equipment and Detection,” Chapter 4 of the book “The Undocumented PC” incorporated by reference above.

If the CPU state is savable, CPU state saver 210 stores one of the CPU state 130A-130N into memory. Then CPU state saver 210 goes from step 212 to step 213 and waits for an event indicating the need for saving CPU state. The state saving event can be, for example, the loading and running of an application program by the operating system or a system interrupt that occurs periodically, e.g. every five second or 10 seconds during the running of an application. The CPU state can be saved more frequently, e.g. by having CPU state on every jump instruction, on every input-output instruction or on every function_call instruction, or some combination depending on flags in e.g. storage element 110D.

On occurrence of such an event, CPU state saver 210 goes to step 215 and analyzes (as described more completely below) the to-be-performed instructions. The CPU state is also saved on occurrence of an error-in which case CPU state saver 210 skips steps 215-215 (described below) and goes directly to step 217. Then, CPU state saver 210 goes to step 216 and injects one or more fix instructions as necessary. For example, a “no-op” instruction can be inserted before a jump instruction for the A0286 microprocessor. As another example, an arithmetic instruction to the PENTIUM can be replaced by a series of arithmetic instructions that work around the error.

Next, CPU state saver 210 enters a critical section in step 217, for example by disabling interrupts and serializing (e.g. making sequential) the execution of instructions in CPU 110A. During this step, if necessary, CPU state saver 210 also switches from main memory 120 to system management memory 130 that is inaccessible to applications 120A-120M.

After entering the critical section, CPU state saver 210 goes to step 218 and stores the CPU state in memory, such as system management memory 130 or main memory 120. Depending on the identity of CPU 110A, one of the save state instructions in Table 1 is used to save the CPU state.

TABLE 1 Address Number for Address Save of Storing for Save CPU State Bytes Current State Identity Instruction OpCode Saved CPU State Software AMD SMI F1h 228 6000:0h FFFFFFF0h 386SXLC (reset) AMD SMI F1h 228 6000:0h FFFFFFF0h 386DXLC (reset) AMD SMI F1h 364 6000:0h FFFFFFF0h 486DXLC (reset) IBM ICEBP F1h 284 6000:0h FFFFFFF0h 386SLC (reset) IBM ICEBP F1h 284 6000:0h FFFFFFF0h 486SLC (reset) Intel Timer 0Fh, 30h; 512 3000:FE00h 3000:8000h 386SL Interrupt note: from enable bit 82360SL 5 in model specific register 1000h Intel Timer 0Fh, 30h; 512 3000:FE00h 3000:8000h 486SL Interrupt note: from enable bit 82360SL 5 in model specific register 1000h

Similarly, a store instruction can also be used to store CPU state in memory. The data saved in a CPU state 130A is illustrated in Table 2 for an INTEL microprocessor (see pages 88-90 of the above-referenced book by Frank van Gilluwe for more details).

TABLE 2 System Total Management Word Memory Offset Stored Data Stored in CPU State FE00 124  Unknown FEF8 2 State Dump Base-This holds the internal register value of the segment for the Suspend code and storage of this table. It defaults to 3000h. FEFC 1 System Management Mode Bits bit 0 = 1 CPU supports I/O Trap restart bit 1 = 1 CPU supports state dump base changes FFFE 1 System Management Mode Revision Number FF00 1 I/O Trap restart-Always set to zero when saved. FF02 1 Halt auto restart-This value is set to 1 if SMI occurred while the CPU was in a HALT instruction. Otherwise it is set to 0. FF04 51  Unknown FFA8 1 ES FFAA 1 Unknown FFAC 1 CS FFAE 1 Unknown FFB0 1 SS FFB2 1 Unknown FFB4 1 DS FFB6 1 Unknown FFB8 1 FS FFBA 1 Unknown FFBC 1 GS FFBE 1 Unknown FFC0 1 LDTR-Local Descriptor Table Register FFC2 1 Unknown FFC4 1 TR-Task Register FFC6 1 Unknown FFC8 2 DR7 FFCC 2 DR6 FFD0 2 EAX FFD4 2 ECX FFD8 2 EDX FFDC 2 EBX FFE0 2 ESP FFE4 2 EBP FFE8 2 ESI FFEC 2 EDI FFF0 2 EIP FFF4 2 EFLAGS FFF8 2 CR0 FFFC 2 CR1

In addition to the above-described contents of storage elements, in this embodiment, CPU state saver 210 also saves application-specific information, such as the name of an application, stack frame information, and operating system specific information, such as the names of all tasks running when the failure occurred and other information of the type saved by Dr. Watson as described above. CPU state saver 210 can also save the contents of a data cache as part of the CPU state. Alternatively, CPU saver 210 can save state on every input-output instruction, thereby to record data operands from as the operands and to CPU 110A. Therefore, the storage elements whose contents are saved are of two types: (1) accessible and (2) inaccessible to the executing software. In this particular embodiment, the CPU state is automatically stored at the memory location 3000:FE00h (FIG. 3) on execution of a save state instruction (Table 1). The current CPU state 130C saved at the address 3000:FE00h is then copied by CPU state saver 210 as shown by arrows 301A-301N to one of CPU states 130A-130N.

If CPU State Saver 210 executes step 218 due to an error, CPU state saver 210 copies all of CPU states 130A-130N to hard disk 115.

Thereafter, CPU state saver 210 goes to step 219 and exits from the critical section, for example by enabling interrupts. In the step 219, CPU state saver 210 also switches from system management memory 130 to main memory 120 as necessary. CPU state saver 210 does not flush a cache memory (not shown) in any of the above-described steps. Thereafter, CPU state saver 210 returns to step 214 and awaits another state saving event.

Although certain specific embodiments have been described herein, numerous modifications and adaptations of the described embodiments will be obvious in view of the disclosure.

For example, instead of proactive debugging of error by CPU state saver 210, errors can also be debugged off-line in a development system, for example by use of an inserted emulator (not shown) to load the saved CPU states 130A-130N (FIG. 2) sequentially into the development system. Such sequential loading of CPU states can recreate the error condition by executing the error-causing software, if all of the necessary data is available, for example in CPU states 130A-130N, or can be recreated by a person skilled in the art of designing debugging systems for microprocessors.

In another variation of one embodiment, when an error has not yet been debugged, an error handler in the software being executed adds an erroneous instruction to the known-to-be-erroneous instructions, with the corresponding temporary-fix instruction to gracefully terminate the application, e.g.. if the erroneous instruction is known to crash (e.g. “freeze”) the CPU. Such graceful termination of an application allows the CPU to continue the execution of other software. Such execution of other software may be valuable to a user, for example by eliminating the need to reboot the operating system that would otherwise have been required in the prior art.

Moreover, any information related to the central processing unit can be saved as a part of the CPU state. For example, if not saved by the above-described save state instruction (Table 1), various CPU registers such as registers CR1, CR2, CR3 and CR4, DR0-DR7 and floating point registers STn, FCS, FSW, FIP, tag word and floating point op code can also be saved as part of the CPU state. The amount of data to be saved in a CPU state is limited only by the size of the system management memory, and the number of saved CPU states required for debugging. If the number of bytes being saved is small, e.g. 228 bytes, several thousand states can be saved in, for example, one MB of system management memory. Alternatively, if several thousands of bytes are saved in a single CPU state (e.g. if data cache is also saved), only a handful of CPU states may be saved in, for example, 32 MB of system management memory.

Numerous such modifications and adaptations of the above described embodiments are encompassed by the attached claims. 

What is claimed is:
 1. A computer process for identifying an error in a central processing unit (CPU), the computer process comprising: executing a first software in said CPU; and repeatedly interrupting said executing of said first software and saving contents of storage elements in said CPU until the error occurs during said executing of said first software; wherein contents saved during each interrupting form a saved state of said CPU; and wherein contents saved during one said interrupting form a currently saved state, said currently saved state being saved in a location different from a location of a previously saved state, so that said currently saved state and said previously saved state form two states in a sequence of states of said CPU maintained by said repeatedly saving.
 2. The computer process of claim 1 further comprising: checking whether the CPU is coupled to a storage media prior to said repeatedly interrupting.
 3. The computer process of claim 1 wherein said repeatedly interrupting is performed periodically.
 4. The computer process of claim 1 wherein a period for performing said repeatedly interrupting is held in a storage element accessible to an operating system.
 5. The computer process of claim 1 wherein said repeatedly interrupting is performed at each jump instruction.
 6. The computer process of claim 1 wherein said repeatedly interrupting is performed at each input-output instruction.
 7. The computer process of claim 1 wherein said repeatedly interrupting is performed periodically and on each function-call instruction.
 8. The computer process of claim 7 wherein said repeatedly interrupting comprises disabling interrupts prior to said saving and enabling interrupts after said saving.
 9. The computer process of claim 8 wherein said repeatedly interrupting comprises serializing execution such that instructions in said first software are executed sequentially.
 10. The computer process of claim 1 wherein said repeatedly interrupting comprises switching to a memory inaccessible during said executing.
 11. The computer process of claim 1 further comprising: automatically terminating execution of the first software on occurrence of the error.
 12. The computer process of claim 1 wherein: said storage elements are inaccessible to said first software.
 13. The computer process of claim 12 wherein: said first software includes an application; and said storage elements are inaccessible to said application.
 14. The computer process of claim 12 wherein: said first software includes an operating system; and said storage elements are inaccessible to said operating system.
 15. The computer process of claim 1 further comprising: saving contents of said storage elements after occurrence of said error, prior to passage of control to a handler for handling said error.
 16. A computer process for identifying an error, the computer process comprising: executing a first software; repeatedly interrupting said executing from a location external to the first software and saving contents of a plurality of first storage elements until an error occurs during said executing, said contents forming a currently saved state and being saved in a location different from a previously saved state, so that said currently saved state and said previously saved state form two states in a sequence of states that are saved by said saving; and loading each state in said sequence of states into a plurality of second storage elements thereby to recreate the error.
 17. The computer process of claim 16 wherein: the first storage elements are internal storage elements of a central processing unit inaccessible to the first software; and the states are saved transparent to the first software.
 18. The computer process of claim 16 wherein: the first software is an operating system, thereby recreating errors in the operating system during said loading.
 19. The computer process of claim 18 wherein: the storage elements are internal storage elements inaccessible to the operating system.
 20. The computer process of claim 18 further comprising: saving contents of the internal storage elements on occurrence of the error, prior to passage of control to a handler for handling the error. 